z3 fake?

(i didn't post this in q & a, because i want to make this an open discussion about fake devices)

the first sony xperia z3 i had was around 70€ cheaper than most of the other z3's online.

the serial number flap in the usb port was missing.

the nfc logo and sony logo on the back were grey and not silver shining.

backlight wasn't same brightness at all parts af the screen (very little).

back glass crack and random screen digitizer crack appeared after 2months.

after had gotten a full replacement after i send it to service, i noticed that this one is faster and the battery also seems to last slightly longer.

bootloader unlock worked without issues over the official sony website.

curstom roms and kernels worked flawlessly.

what was going on with my past one?

how can it be unlocked by sony and install kernels and roms, but yet seems to be fake, and sony replaces it?